Underground storage is safer than storage in spheres on the surface, both with respect to vulnerability to acts of sabotage or aircraft crashes and also with respect to protecting the air and the water-bearing environment in the event of a breakage. The basic design consists in digging underground cavities in rock, and in sealing the storage volume by means of a waterproof membrane.
Numerous underground storage installations exist for petroleum products such as gasoline, diesel oil, heating oil, and liquefied gases such as butane or propane. These products are immiscible with water. If any water does flow into the mass of stored product, it does not mix with the product, but rather it collects in the bottom of the cavity. This is no great handicap and frequently takes place in many storage installations. However, when the stored product is ammonia, the presence of water reduces the concentration of the ammonia which becomes unuseable even with very small quantities of water (a few percent).
Thus with petroleum products it is common practice to provide storage installations in which the surrounding water pressure is greater than the pressure of the product in the cavity. The water which flows into the cavity is easily pumped out from a sump in the cavity, and there is no risk of the petroleum product escaping into the surrounding formations. With ammonia or the like, the conditions are quite different.
Ammonia (NH.sub.3) boils at -33.4.degree. C. at atmospheric pressure. It can thus be stored under such conditions. However, such cryogenic storage is very expensive to set up and has increased running and maintenance costs compared with pressurized storage. Ammonia is easily liquefied under pressure, with the following boiling points at various pressures:
+2.degree. C. at 4 bars; 15.degree. C. at 6 bars; 23.degree. C. at 8 bars. An underground gallery at a depth of several tens of meters is at a temperature of about 15.degree. C., and the present invention envisages storage taking place under such conditions. Clearly the temperature may depart temporarily from this value, eg. immediately after storing ammonia at some other temperature. However, in order to avoid running the risk of freezing water in the surrounding formations, ammonia must not be stored at a temperature of 0.degree. C. or less since the resulting deformation could damage the installation.
Given these characteristics of ammonia, together with its chemical properties, in particular in relation to water, a storage installation designed for ammonia must cope with two problems:
(1) the stored ammonia must be completely confined in the storage volume, ie. there must be not outward leaks (to the atmosphere or to the water in surrounding formations); and
(2) any water in the surrounding formation (whether from the water table or from intermittent infiltration through unsaturated rock) must not be able to penetrate the storage volume in quantities which could degrade the product.
The design of the storage installation must take account of both criteria for liquid-tightness, and these criteria must continue to be satisfied for the life of the installation. The design may be based on interposing a completely impermeable membrane between the product and the surrounding medium. The membrane must be strong enough to withstand the internal pressure of the stored ammonia even in the event of changes in shape or size (eg. due to a change in the temperature of the stored product, or to a crack in the encasing concrete). The membrane must also withstand internal corrosion from the product and external corrosion from the surrounding medium (water).
Advantageously, the impermeable membrane (or envelope) is made of metal. As a general rule, most metals and their alloys are essentially inert with respect to ammonia and are not subject to generalized corrosion therefrom. However, copper and zinc and their alloys must not be used, particularly in the presence of water. Finally, oxygen and nitrogen, and in more general terms air, can lead to a corrosion phenomenon of construction steels under stress. Adding a small amount of water (0.2% by weight according to legislation currently in force in the USA) to the ammonia appears to eliminate this risk. In conclusion, the use of mild steel is recommended, provided the welding is properly done and provided that 2.degree./oo of water may be added to the product.
In the method described in the present assignee's published French patent specification No. 2 380 200 dated 8th Mar. 1977, water is maintained around the envelope at a higher pressure than the internal pressure. Contamination of underground water is thus avoided, but the possibility of contaminating the stored product is not avoided, supposing that a leak were to appear in the envelope.
Preferred implementations of the present invention provide a method and apparatus in which there is no danger of contamination, and in which any leak through the envelope can be detected immediately. The method and the apparatus of the present invention are described below in relation to ammonia, but it will be understood that they are applicable to storing products having analogous or related characteristics.